Shoutbox

Frictionless just means there's no counterforce along (i.e. preventing it from sliding across the surface), both Chancer and blessedguy already incorporated that into their models.

Regardless of friction, a component of the gravitational force on the object is into the sloping surface and similarly the ground is exerting an equal force onto the object at an angle perpendicular to the surface (see Wcos(A) in Chancer's model) which acts to cancel out a part of the gravitational force.


Probably a bit more intuitive to think of a practical example:
A man who is skating across ice which is angled ever so slightly downwards will not be accelerating at anywhere even remotely near the same rate at which a man who fell through the ice (or jumped out of a plane) would be.


The only difference friction would make is that there would be an additional force acting in the same direction as T (the tension on the rope in Chancer's model) i.e. across the surface, preventing it from sliding -- which as you can see has been omitted in his model :)

PS: I know im terrible at explaining stuff :P

One last go: a portion of the gravitational force is into the surface which is completely counteracted by equal+opposite force (stopping it from falling through the object). Therefore the only remaining net force is that portion of the gravitational force which remains.
The lesser the slope, the greater the component of the gravitational force which is counteracted (as there is a greater portion being applied into the surface).